1. Field of the Invention
This application relates to methods for depositing heteroepitaxial films in semiconductor manufacturing. More particularly, this application relates to methods for depositing silicon germanium heteroepitaxial films by using silylgermane as a source gas.
2. Description of the Related Art
Silicon germanium (SiGe) films are used in a wide variety of semiconductor applications, including microelectronic devices. One issue that often arises is how to increase the amount of “strain” in an active layer of a device, which leads to increased device performance. A deposited epitaxial layer is said to be “strained” when it is constrained to have a lattice structure in at least two dimensions that is the same as that of the underlying single crystal substrate, but different from its inherent lattice constant. Lattice strain occurs because the atoms in the deposited film depart from the positions that they would normally occupy in the lattice structure of the free-standing, bulk material when the film deposits in such a way that its lattice structure matches that of the underlying single crystal substrate. For example, greater amounts of germanium in an epitaxial layer deposited onto a single crystal silicon substrate or layer generally increase the amount of compressive strain. This strain occurs because germanium in its pure form has a 4% greater lattice constant compared to silicon. Therefore, the higher the germanium content, the greater the lattice mismatch with the underlying silicon.
Strain is in general a desirable attribute for active device layers, since it tends to increase the mobility of electrical carriers and thus increase device speed. In order to produce strained layers on conventional silicon structures, however, it is often helpful to create strain-free, intermediate heteroepitaxial layers for further strained layers that will serve as active layers in semiconductor devices. These intermediate films often comprise a relaxed silicon germanium “buffer” layer over a single crystal semiconductor substrate or wafer surface.
To be an effective buffer layer, the silicon germanium layer should be relaxed. When the thickness of the silicon germanium buffer layer increases above a certain thickness, called the “critical thickness,” the buffer layer begins to relax to its inherent lattice constant, which requires the formation of misfit dislocations at the film/substrate interface. The critical thickness depends on a variety of factors, including growth rate, growth temperature, germanium concentration, and the number of defects within the layer underlying the silicon germanium layer. Once the silicon germanium layer reaches its critical thickness, the layer begins to relax toward its own lattice size and strain is reduced. A silicon germanium layer that has adopted its own natural lattice constant on a nanometer scale through the film may be considered fully relaxed.
Depositing silicon germanium films presents several challenges. Often, it is difficult to achieve a high germanium concentration, whether for maintaining strain or relaxing for use as a buffer, without excessive faults that interfere with electrical operation of the integrated circuit. In addition, it is often difficult to deposit silicon germanium films that are thin and smooth, particularly at low temperatures, to both conserve thermal budget and maintain flatness for integration with subsequent processing. Although numerous source gases are available to create silicon germanium layers, each has its limitations. For example, conventional source gases (e.g. silane) are unable to deposit smooth homogeneous films of low thickness at low temperatures. These source gases often result in films with high surface roughness due to nucleation islands.
More exotic source gases, such as SiH3GeH3, (H3Ge)2SiH2, (H3Ge)3SiH and (H3Ge)4Si have been used to deposit silicon germanium layers. One of the perceived advantages for such gases, containing both germanium and silicon, is that the intrinsic ratio of Si:Ge within the compound is thought to strongly influence the ratio of Si:Ge in the deposited film, permitting wider process windows for a given desired ratio. However, these source gases are costly, lack the flexibility of conventional precursors, and often entail significant hardware adjustments and tuning that have not generally been found worthwhile compared to the perceived benefits of these precursors.